Image dissector camera tube with electronically variable selector aperture



Sept. 23, 1969 H ET AL 3,469,141

IMAGE DISSECTOR CAMERA TUBE WITH ELECTRONICALLY VARIABLE SELECTOR APERTURE Filed Jan. 51, 1968 NIH WU 14! I 1 I I I I FIG. 2

Q BY2 ATTORN Y United States Patent Ofi ice IMAGE DISSECTOR CAMERA TUBE WITH ELEC- TRONICALLY VARIABLE SELECTOR APERTURE James A. Hall and James F. Nicholson, Severna Park, Md., assignors to Westinghouse Electric Corporation,

Pittsburgh, Pa., a corporation of Pennsylvania Filed Jan. 31, 1968, Ser. No. 701,966

Int. Cl. H01j 31/26 US. Cl. 315- 4 Claims ABSTRACT OF THE DISCLOSURE BRIEF SUMMARY OF THE INVENTION This invention relates to image dissector camera tubes which are a photosensitive electronic imaging device. The image dissector tube incorporates a photocathode onto which the input radiations are directed and the photoelectrons emitted from the photocathode are focused onto and scanned over a plane containing a small aperture. This aperture samples the electron current from a small area or portion of a photocathode. By the inclusion of the deflection mechanism, the sampling can be changed sequentially from one photocathode area to the next. In this manner, the selecting aperture monitors the entire image area on the photocathode. The electrons after passing through the aperture enter an electron multiplier. The output from the electron multiplier may be then fed into a conventional amplifier and used as a'video signal to modulate a cathode ray tube output or any other well known video system. Thus an image is reconstructed of the input light image which is focused onto the photocathode.

The image dissector tube is one of the early electronic pickup tubes and is described on pages 729 and 730 in the book entitled Vacuum Tubes by Karl R. Spangenberg and published in 1948 by the McGraw-Hill Book Company Inc. It is found that the image dissector tube has a resolution capability limited only by the size of the selector aperture. Unlike other television camera tubes, wherein the resolution defining element has a gaussian sensitivity profile, the magnetically focused dissector tube has an essentially uniform response across the aperture image. Thus, tubes of very high resolution capability may and have been built by making the selector aperture substantially smaller, provided there is enough light available to produce an acceptable picture. Unfortunately, reducing the size of the selector aperture area proportionately reduces the sensitivity of the tube, that is, the illumination level required for a given signal to noise ratio increases. The signal to noise ratio is determined primarily by the shot noise present in the photoelectrons collected from the image element.

A variable selector aperture may be obtained by either changing its physical dimensions or by changing the electron trajectories within the tube so as to concentrate more or less current into the aperture. To change the physical dimensions of the aperture when located inside an evacuated envelope is difficult. A more attractive alternative is to keep the same physical aperture but to change 3,469,141 Patented Sept. 23, 1969 its effective size by altering the focusing field conditions. The trajectory adjustment may be made, either in the image section before the electrons reach the deflection field, or later after they leave the deflection -field. A small amount of adjustment in the image section of the tube can be made with an arrangement similar to that used in a zoom camera tube. However, to obtain a large range of variation, a complex electron optical arrangement is needed and a change in image size may result from image section adjustment. Furthermore, it is considered desirable from an operational viewpoint to obtain a required change in aperture size by a variation in one electrode potential. The introduction of a small electrostatic lens immediately prior to the selector aperture does not suffer from the above difliculties and offers the simplest method of aperture size variation.

BRIEF DESCRIPTION OF THE DRAWING DETAILED DESCRIPTION Referring now to FIGURE 1, there is illustrated an image dissector tube. The tube comprises an envelope 10. The envelope 10 includes a tubular neck portion 12 or substantially uniform diameter with an image portion 14 which may be of slightly larger diameter. The image section 14 is provided with an end wall or input window 16 having a photocathode 18 provided on the inner surface. The photocathode 18 may be of any suitable material such as cesium antimony. The material in the photocathode 18 is responsive to visible light radiations directed onto the photocathode 18 and generates electrons. A diaphragm or aperture plate 20 is provided in the neck portion 12 and has a selecting aperture 22 provided therein. The photoelectrons emitted from the photocathode 18 are accelerated by tubular electrodes 24, 26 and 28 located in the image section 14. Suitable voltages for the electrodes 24, 26 and 28 may be 2700 volts, 2400 volts and 2100 volts respectively negative with respect to ground. The photocathode 18 is connected to a suitable voltage source 30 such as a battery so that the photocathode 18 is at a potential of about a negative 3000 volts with respect to ground. The aperture plate 20 is also connected to a suitable potential source 32 which may be of a potential of about a negative 1800 volts with respect to ground. A conductive coating 33 may also be provided on the inner surface of the neck portion 10. The coating 33 would be at a negative 1800 volts.

The electrons emitted from the photocathode 18 are focused onto the plane of the aperture plate 20' by means of a focus coil 34. The focus coil 34 provides substantially a constant magnetic field and the flux lines are essentially parallel to the tube axis. A deflection yoke 36 is also provided between the photocathode 18 and the aperture plate 20 exterior of the envelope 10 for deflecting the electron beam emitted from the photocathode 18. This yoke is significantly shorter than the distance between cathode 18 and plate 20. For this reason the electrons emitted from the photocathode 18 and directed onto the aperture plate 20 after undergoing deflection approach the plate 20 substantially parallel to the axis of the envelope 10. Suitable currents are applied to the deflection yoke 36 so as to scan the electron beam from the entire image area of the photocathode 18 at some time across the aperture 22 in the aperture plate 20.

Positioned on the opposite side of the aperture plate 20 with respect to the photocathode 18 is a suitable electron multiplication structure 40 which consists of a plurality of secondary emissive dynodes 41 for amplifying the electron current passing through the aperture 22 in the aperture plate 20. An anode 42 is provided at the exit end of the multiplier 40 for collecting the electrons. The anode 42 is normally operated at a potential near ground while the secondary emissive dynodes 41 in the electron multiplier 40 are provided with less negative voltages from the aperture plate 20 which is at a negative 1800 volts to the anode 42 which is at a potential of approximately ground.

The above described structure is substantially that of a conventional image dissector tube as is well known in the art. In the subject invention, an electrostatic lens 49 is positioned in front of the aperture 22 in the aperture plate 20. The specific structure shown herein consists of three cylinders 50, 52 and 54 coaxially positioned within the tube on the axis passing through the aperture 22. In a specific example shown the middle cylinder 52 is of greater diameter than the two outer cylindrical electrodes 50 and 54. The outer electrodes 50 and 54 are connected together electrically and are also connected electrically to the aperture plate 20 so as to operate at a similar potential. The electrode 52 is connected to a variable voltage source 56. The voltage source 56 is a variable voltage source providing a potential of about 1800 volts negative with respect to ground which may be varied so as to eflect desired focusing. In the specific example shown the diameter of the aperture 22 may be about .010 to .001 inch depending on resolution desired with the length of the lens 49 consisting of the electrodes 50, 52 and 54 about one inch or about ,5 the distance between the photocathode 18 and the aperture plate 20-. The inside diameters of the cylinders 50 and 54 may be about A to inch. The center electrode 52 may have its potential varied from approximately negative 1000 volts to a negative 2400 volts for best focus. It is important to keep the lens structure short in order to avoid disturbing the field within the tube and especially to avoid altering deflection geometry and also to stay within the region where the incoming electron paths are substantially paraxial.

It is also obvious that the geometry of the tube is not critical and the electronic lens 49 may be used in an image dissector with electrostatic deflection and mag netic focusing or with only electrostatic focusing and deflection. It is only necessary that the electron paths prior to the aperture 22 are substantially parallel to the axis. It is also obvious that any other suitable electronic lens may be utilized in front of the aperture 22 to provide the desired focusing. The effect of the electron lens 49 is of course to increase the number of electrons entering the aperture 22 which in effect increases the sensitivity of the tube. If the lens 49 is set up so that it decreases the number of electrons entering the aperture over that without the electron lens then the result is that the resolution of the tube is increased. By this electro static lens, one is able to either improve the resolution or improve the sensitivity by variation of the potential source 56.

While the preferred embodiment of the invention has been disclosed herein, it is obvious that other modifications are feasible within the spirit and scope of the inventlon.

We claim as our invention:

1. An image dissector comprising an evacuated envelope having therein a photocathode for transforming a radiation image into an electron image corresponding to said radiation image, an electron multiplying means disposed to receive the flow of electrons from said photocathode, a diaphragm disposed between said photocathode and said electron multiplying means and having an aperture through which a portion of said flow of electrons is directed to said electron multiplying means, focusing means provided for focusing the electrons emitted from said photocathode onto said diaphragm, deflection means for deflecting said electron image over said aperture to selectively permit portions of the electron image from said photocathode to pass through the aperture in said diaphragm, an electrostatic lens means provided between said photocathode and said diaphragm and in front of the aperture in said diaphragm and intercepting only a portion of said electron image and extending for a distance therefrom of less than of the distance between said photocathode and said diaphragm, said electrostatic lens means varying the flow of electrons through said aperture to efiect a change in the apparent size of said aperture.

2. An image dissector comprising an evacuated envelope having therein a photocathode for transforming a radiation image into an electron image corresponding to said radiation image, an electron multiplying means disposed to receive the flow of electrons from said photocathode, a diaphragm disposed between said photocathode and said electron multiplying means and having an aperture through which a portion of said flow of electrons is directed to said electron multiplying means, focusing means provided for focusing the electrons emitted from said photocathode onto said diaphragm, deflection means for deflecting said electron image over said aperture to selectively permit portions of the electron image from said photocathode to pass through the aperture in said diaphragm, an Einzel type electrostatic lens provided between said photocathode and said diaphragm and in front of the aperture in said diaphragm and intercepting only a portion of said electron image, said Einzel type of lens having its outside electrodes operating at a potential substantially the same as said diaphragm and the intermediate electrode of said Einzel lens operating at a different potential with respect to the outside electrodes, said Einzel lens providing means for varying the flow of electrons through said aperture to effect a change in the apparent size of said aperture.

3. The image dissector defined in claim 2 in which said intermediate electrode is connected to a variable potential source to vary in effect the aperture size in said diaphragm to provide either an improvement in resolution or an improvement in sensitivity of said image dissector tube.

4. An image dissector comprising an evacuated envelope having therein a photocathode for transforming a radiation image into an electron image corresponding to said radiation image, an electron multiplying means disposed to receive the flow of electrons from said photocathode, a diaphragm disposed between said photocathode and said electron multiplying means and having an aperture through which a portion of said flow of electrons is directed to said electron multiplying means, focusing means provided for focusing the electrons emitted from said photocathode onto said diaphragm, deflection means for deflecting said electron image over said aperture to selectively permit portions of the electron image from sald photocathode to pass through the aperture in said diaphragm, an electrostatic lens means provided between said photocathode and said diaphragm and in front of the aperture in said diaphragm and intercepting only a portion of said electron image, said electrostatic lens including at least one electrode provided adjacent the aperture in said diaphragm in which the electrode of said electrostatic lens facing said photocathode operates at a potential substantially the same as said diaphragm to thereby avoid disturbances of the field between said photocathode and said electrostatic lens.

References Cited UNITED STATES PATENTS 3,366,830 1/1968 Polye 3l5ll RODNEY D. BENNETT, JR., Primary Examiner C. E. WANDS, Assistant Examiner 

